1. Field of the Invention
The present invention is directed to an electrosurgical guidewire. More particularly, the invention is directed to a guidewire which is energized by radio frequency current to penetrate occlusive tissue so as to permit the subsequent passage of a therapeutic device for treatment.
2. Description of the Prior Art
In the U.S., heart disease, stroke and related disorders account for almost as many deaths as all other causes of death combined. The single largest cause of cardiovascular diseases is sclerosis--a build-up of fatty or calcific deposits in the arterial lumen. These deposits can impair, and in severe cases, totally obstruct the flow of blood. A number of medical devices are designed to displace, disperse or extract the occlusive tissue. However, most of these devices operate over or conjunction with a guidewire.
Initial placement of the guidewire is a problem in cases of total occlusion. Although some occlusions can be negotiated by forcible advancement of a blunt catheter (the Dotter technique), or by rotational means (orthogonal displacement of friction), a reliable method is required which will unconditionally traverse a total occlusion to permit guidewire insertion and subsequent passage of a therapeutic device for treatment.
It is well known that electrical currents are able to flow in tissue because of the presence of free electrons and ions in solution within the tissue structure. Many physiological functions are made possible by the electrical conductivity of the body. Passage of the electrical current in the mammalian or similar bodies dissipates some energy in the form of heat because of the finite electrical resistance of the body tissue (power density equalling resistivity "ohm-cm" times the square of current density (amps/cm.sup.2). At low current density values, this product of heat is totally insignificant. If the current density levels are increased, it is possible to vaporize the entrained tissue water. If this vaporization occurs with sufficient speed before the water vapor can diffuse through the tissue, it will locally rupture and incise tissue. Surgically producing such ruptures in a linear fashion is the essence of electrosurgery.
In order not to cause neuromuscular stimulation in electrosurgery, currents at frequencies in excess of 100 kilohertz are used. At these frequencies the muscle fibers are unable to respond. As electromagnetic radiation in this frequency range is used for the broadcast of radio signals, electrical currents in this frequency range are referred to as radio frequency, or RF. Electrosurgery is typically accomplished by passing RF current through a small electrode in the form of a scalpel or needle into the tissue and completing the electrical circuit through the tissue by attaching a much larger electrode plate (indifferent electrode or patient plate) elsewhere on the body. The small electrode in the form of a scalpel or needle is referred to as the active electrode because it is where the surgical effect occurs due to the much higher current density adjacent to the small area of the electrode. Since the total current at any place in the circuit is equal to the total current at any other place in the circuit at any instant of time, the current density (i.e., current divided by area) at the relatively large patient plate is extremely low, so as not to cause any noticeable effect on the patient. Conductive jellies are used to assure good conductance from the patient's body through the patient plate and to avoid hot spots and burns which would result from reduced current flow areas and high current densities.
Under certain circumstances, the patient plate or indifferent electrode can be eliminated. If the patient plate electrode circuit connected to the console line cord ground wirer the electrical energy can be coupled out of the patient's body capacitively to earth ground. Connecting the patient plate electrode circuit to the console line ground places the active electrode at high potential relative to the earth and the building, and thus the patient. The inconvenience of the patient plate is eliminated, and there is no risk of burns at the plate. Although electrosurgery can be performed using RF frequencies from 300 kilohertz to 3 megahertz, the stray capacitance technique benefits from the use of higher frequencies, since a stray capacitance at a given value would effect a lower impedance at a higher frequency. This is based upon the relationship whereby the reactance of a given capacitor is inversely proportional to frequency.
Electrosurgery using a single active electrode is properly defined as monopolar electrosurgery. Monopolar scalpels are commonly referred to as "Bovies" in honor of the person who popularized their use.